Loudness of time-varying stimuli with electric stimulation

Applications of Phenomenological Loudness Models to Cochlear Implants

Cochlear implants electrically stimulate surviving auditory neurons in the cochlea to provide severely or profoundly deaf people with access to hearing. Signal processing strategies derive frequency-specific information from the acoustic signal and code amplitude changes in frequency bands onto amplitude changes of current pulses emitted by the tonotopically arranged intracochlear electrodes. This article first describes how parameters of the electrical stimulation influence the loudness evoked and then summarizes two different phenomenological models developed by McKay and colleagues that have been used to explain psychophysical effects of stimulus parameters on loudness, detection, and modulation detection. The Temporal Model is applied to single-electrode stimuli and integrates cochlear neural excitation using a central temporal integration window analogous to that used in models of normal hearing. Perceptual decisions are made using decision criteria applied to the output of the integrator. By fitting the model parameters to a variety of psychophysical data, inferences can be made about how electrical stimulus parameters influence neural excitation in the cochlea. The Detailed Model is applied to multi-electrode stimuli, and includes effects of electrode interaction at a cochlear level and a transform between integrated excitation and specific loudness. The Practical Method of loudness estimation is a simplification of the Detailed Model and can be used to estimate the relative loudness of any multi-electrode pulsatile stimuli without the need to model excitation at the cochlear level. Clinical applications of these models to novel sound processing strategies are described.

Perception and prediction of loudness in sound coding strategies using simultaneous electric stimulation

Cochlear Implant (CI) sound coding strategies based on simultaneous stimulation lead to an increased loudness percept when compared to sequential stimulation using the same current levels. This is due to loudness summation as a result of channel interactions. Studying the loudness perception evoked by dual-channels compared to single-channels can be useful to optimize sound coding strategies that use simultaneous current pulses. Fourteen users of HiRes90k implants and one user of a CII implant loudness balanced single-channel to dual-channel stimuli with varying distance between simultaneous channels. In this study each component of a dual channel was a virtual channel, which shared current across two adjacent electrodes. Balancing was performed at threshold and comfortable level, for two spatial references (apical and basal) and for dual-channels with different relative current ratios. Increasing distance between dual-channels decreased the amount of current compensation in the dual-channel required to reach equal loudness to a single channel component by an average of 0.24 dB / mm without a significant difference between threshold and most comfortable level. If the components of the dual-channels were not at equal loudness, the loudness summation was reduced with respect to the equal loudness case. The results were incorporated into an existing loudness model by McKay et al. (2003). The predictions from the adapted model were evaluated by comparing the loudness evoked by simultaneous and sequential sound coding strategies. The application of the adapted model resulted in a deviation between predicted and actual behavioral loudness balancing adjustments in electrical level between simultaneous and sequential processing strategies of 0.24 dB on average.

Optimal gain control step sizes for bimodal stimulation

OBJECTIVE

Cochlear implants (CI) and hearing aids (HA) have a gain control that allows the bimodal user to change the loudness. Due to differences in dynamic range between CI and HA, an equal change of the gains of the two devices results in different changes in loudness. The objective was to relate and individualise the step sizes of the loudness controls to obtain a similar perceptual effect in the two ears.

DESIGN

We used loudness models parametrised for individual users to find a relation between the controls of the CI and the HA such that each step resulted in an equal change in loudness. We conducted loudness balancing experiments to validate the results.

STUDY SAMPLE

Eleven bimodal users of whom six were tested in a prior study.

RESULTS

The difference between the optimal gain from the loudness balancing procedure and actual gain was 3.3 dB when the new relation was applied. In contrast, the difference was 8 dB if equal step sized were applied at both sides.

CONCLUSION

We can relate the controls such that each step results in a similar loudness difference.